The present invention relates to a metal palladium—or alloy palladium—composite membrane which exhibits a high permeable hydrogen flux and high hydrogen selectivity for separation or purification. The present invention further relates to a preparation method for producing these said metal or alloy palladium composite membranes.
The increased demand for hydrogen in recent years in many sectors such as petroleum refining, petrochemical and semi-conductor processing and in new energy-related applications such as clean fuel for fuel cells and vehicles has led to a high interest in methods for separation and purification of hydrogen from gas mixtures. Palladium membranes or alloy palladium membranes have been the subject of many studies, due largely to their unmatched potential as hydrogen selective membranes for separation or purification and membrane reactors for hydrogenation/dehydrogenation reactions. However, these studies show that improvement regarding hydrogen permeability in pure and composite membranes is necessary, as the measured hydrogen permeation flux throughout the studies remained very low.
Another significant problem found in membrane studies is, under 275° C., pure palladium membrane is susceptible to cracking during the phase transformation due to the amount of hydrogen absorbed. This phenomenon is often termed ‘hydrogen embrittlement’. It is known however that hydrogen embrittlement can be dramatically reduced, even at room temperature, by the formation of an alloy palladium membrane such as Pd—Ag alloy membrane, Pd—Cu alloy membrane or Pd—Au alloy membrane.
Membranes can either exist as a pure or as a composite membrane. A pure palladium membrane, that is an unsupported palladium membrane, is well known in the public domain. The corresponding thickness of the membrane must be greater than 200 microns in order to ensure enough mechanical strength to sustain the reaction; consequently this specified membrane thickness results in a low hydrogen permeable flux and a very high production cost. Therefore composite membranes are preferred, that is a supported palladium membrane where the palladium membrane layer is connected to a porous substrate support, hence reducing the required thickness of the palladium membrane layer and consequently reducing the expense and improving the hydrogen permeation flux, whilst still maintaining the mechanical strength of the whole membrane.
As shown in the prior art, conventional composite membranes exist in three defined layers; palladium/palladium alloy membrane layer, the surface substrate support and an intermediate layer interconnecting the previous two layers. Indeed, in order to obtain a thin palladium/palladium alloy membrane layer on porous substrates, an intermediate layer is usually introduced in the prior art, which results in a three-layer composite membrane. However this three layer formation again presents the major problem of a low hydrogen permeation flux.